Martensitic stainless steels are typically in the lower range of chromium for stainless steels and therefore lower in corrosion resistance compared to the other stainless steels. Martensitic stainless steels can be heat treated to a wide range of strengths and have good machinability when sulfur is added and the steels are in the heat treated condition. Martensitic stainless steels are usually easy to heat treat and relatively easy to hot and cold work. Typically, the martensitic stainless steels are heated to a high temperature, such as 1700.degree. to 2000.degree. F. (930.degree. to 1095.degree. C.) and then air or oil quenched. A second heat treating step from 800.degree. to 1400.degree. F. (425.degree. to 760.degree. C.) tempers the martensitic stainless to the desired strength level. Martensitic stainless steels generally tend to be lowest cost of all the stainless steels.
Materials used for manufacturing golf club heads have varied considerably over the past several decades. Stainless steels, carbon steels and many other alloys have been used for golf club heads to provide the desired combination of hardness, weight, ductility, corrosion resistance, strength, toughness, abrasion resistance, wear resistance and resilience. Various alloys have also been used for the shaft of golf clubs which may have different property requirements than the heads of the golf clubs.
The alloys used for golf club heads were initially well known materials used in sand and investment foundries for casting. Other club manufacturers have chosen to go the route of forged clubs which require more finishing work. Familiarity and availability were the main reasons many of the foundries used specific stainless steel alloys rather than designing a composition for the golf head properties. Recently, club designers have experimented with new unusual alloys which were more expensive but offered specific properties, such as better feel or hardness. The properties of the various alloys for golf clubs were also modified by heat treatments to develop increased hardness or strength.
When it comes to the selection of a club material, some manufacturers have spared no expense if the club can provide added feel or distance for the golfer. More expensive alloys such as copper-beryllium, copper-tin, copper-nickel-zinc and aluminum-titanium have been used as well as surfaces having a composite structure with fibers impregnated.
Golf club heads may be forged or cast. The use of investment cast heads allows the club manufacturers to purchase detailed castings which require no or minimal finishing operations. The freedom in design is greatly increased with the use of castings. Casting tooling includes the hosel detail, scoring lines and identification as part of the mold. Forged clubs are more limited in design and require considerable finishing operations. Forging tooling is far more expensive if club design changes are required. Forged articles generally would have a higher density because of the working of the material. The amount of forging reduction has a strong influence on the metallurgical structure. Forgings may also be produced at manufacturing plants which do not have melting or casting equipment. The properties requirements for golf club heads permit either cast or forged production.
The selection of a material for a golf club head must consider many properties. The finished head weight must fall within very narrow limits to comply with specifications. The metal must be capable of withstanding the wear and impact forces associated with playing the game. The tensile strength, fracture resistance, hardness and density of a material must all be considered in selecting a material for casting.
Stainless steels are used for golf clubs because they provide the above properties and also have excellent corrosion resistance. The most common choices of stainless steels have been T304, T431 and 17-4PH. Each of these materials offers different properties.
T304 is an austenitic material having about 18% Cr, 8% Ni and less than 0.08% C. This stainless is relatively soft and can not be hardened by heat treatment. While very corrosion resistant, its use is restricted to irons having thicker hosel bases which helps to limit the amount of bending. Austenitic stainless steels, such as T304, have been used but tend to mar very easily. Often these steels were selected because scrap was available at a reasonable price. The austenitic stainless steels have a large addition of nickel which greatly increases the cost of the material. The lower strength level as cast does not allow a more streamlined golf head design to be used.
The 400 series of stainless steels has also been used to provide the desired hardness and corrosion resistance for golf clubs. However, these alloys require a suitable heat treatment and close control of chemistry to achieve the desired properties. Type 431 is commonly used and requires a double heat treatment to obtain the desired properties. The steel lacks the ductility required for adjusting the alignment of the head and the hosel. T431 is a martensitic stainless consisting of about 16% Cr, 2% Ni and less than 0.2% C. It is less corrosion resistant than T304 and is usually given a passivation treatment to clean the surface. T431 can be heat treated to provide high strength and hardness levels and is restricted for use in wedges, putters and ironheads.
The stainless steel widely used for golf clubs has been 17-4 PH (see U.S. Pat. Nos. 2,482,096; 2,482,097 and 2,482,098). It has the desired corrosion resistance and a hardness in the Rockwell C range of about 30 to 35. It can not be sofened to a significantly lower level to obtain the desired feel when striking the ball. This steel was designed originally for aircraft requirements and was not designed for the properties needed for the golfing industry. Many golf club heads have been designed using 17-4 PH steel simply because it is well known, available as remelt stock and is forgiving of minor chemistry variations. 17-4 PH is a precipitation hardenable steel having about 17% Cr, 4% Ni, 2.75% Cu and less than 0.07% C. It is the strongest and hardest of the stainless steels presently used for this application.
Some club designers have used chromium plated clubs but these tend to show corrosion when dinged.
One alloy designed specifically for the golfing industry is described in U.S. Pat. No. 4,314,863 by Jon McCormick of Fansteel Inc. (incorporated by reference). The stainless steel casting alloy consisted of 13 to 19% chromium, 2 to 3.6% nickel, 2 to 3.5% copper, 0.20 to 1.4% manganese, 0.5 to 1.0% silicon, 0.1 to 0.8% carbon, 0.10% max nitrogen, less than 0.10% molybdenum, less than 0.10% aluminum, less than 0.10% columbium, 0.035% max sulfur, 0.035% max phosphorus and balance essentially iron. The sum of nickel and copper must be at least 5%. The stainless casting was designed to be economical, to provide the desired hardness of about Rockwell B 90 and to provide other mechanical properties without requiring any supplemental heat treatments. The preferred microstructure is substantially austenite in combination with some martensite or delta ferrite.
Another stainless steel developed for the golf club head industry is disclosed in Japanese publication J55029329. The alloy is designed to produce good vibration dampening and has a composition comprising 8-25% Cr, 0.2-3.0% Mo, 0.5-3.0% Ni, 1.0-4.0% Si, 0.06% maxC, and balance Fe. The typical alloy had about 18% Cr, 1% Mo, 1% Ni, 2.5% Si, 0.005% C and balance Fe. The main improvements in dampening were attributed to the additions of Cr and Mo.
Stainless steels are widely used in marine applications because of their excellent corrosion resistance. Alloys such as T431, 15-5 PH, and 17-4 PH are widely used for applications such as boat propellers. Marine applications also require alloys which have good ductility, strength and hardness. However, the PH alloys are over-graded for these uses and there exists a need for a more cost effective and easier to heat treat alloy.
Martensitic stainless steels have been developed for the marine industry which possess good pitting resistance and high strength. An example is Japanese publication J 01246343 which comprises up to 0.08% C, up to 3% Si, up to 3% Mn, 2.5-5.0% Cu, 2.5-6.0% Ni, 10.0-20.0% Cr, 1.5-5.0% Mo, 0.1-1.0% Nb and/or Ta, 0.005-0.050% B, 0.105-0.40% N and balance Fe. The alloy was for use as marine pumps, shafts and valves.
Another martensitic stainless for marine applications is represented by Japanese publication J 63000436. The steel comprises 0.03% max C, 0.30-0.60% Si, 0.7-1.00% Mn, 0.15-0.45% Ni, 11.5-12.5% Cr, 0.5% max Mo, 0.30-0.50% Cu, 0.060% N and balance Fe. The alloy has good welding characteristics including the capability of being welded without preheating.
None of the alloys presently used for golf clubs have the desired combination of properties to be capable of providing the complete production of all of the desired clubs and designs. Furthermore, the expense of the materials and the cost of the required heat treatments or finishing steps results in the need for a more economical alloy with the desired range of properties. The existing metals used for the manufacture of golf club heads are expensive and deficient in one or more properties and have additional processing steps required to enable its use.